HiWaQ v1.0: A flexible catchment water quality assessment tool with compatibility for multiple hydrological model structures

Abstract. Mitigating diffuse-source nutrient pollution has created an urgent need to advance understanding of catchment hydrological and nutrient dynamics, and develop robust integrated hydrological and water quality models to support decision making. However, the current availability of integrated catchment-scale water quantity and quality assessment tools is very limited compared to that of hydrological models, and the common developing strategy of extending existing hydrological platforms might be restricted by specific hydrological structures and associated data requirements. Here we introduce a new flexible catchment water quality assessment tool – HiWaQ that aims to be compatible with multiple, often contrasting hydrological model structures and that comprehensively considers spatio-temporally varying water quality impacts of anthropogenic activities. The flexibility of HiWaQ is realised through: (1) a unified configuration interface for catchment characteristics and the coupled hydrological structure; and (2) a generalised structure of storage-flux interactions for all bucket-type storages. We also present the detailed N module development (HiWaQ-N) for nitrate simulation and its coupling tests with two contrasting fully distributed hydrological models (the process-based ecohydrological EcH₂O-iso model and the multi-scale conceptual mHM model). The two couplings were tested and cross-compared in the mixed forest-agricultural Silberhütte catchment (99 km²), central Germany, using the continuous daily discharge and Nitrate-N observations over 2012–2018. Results demonstrated that: (1) HiWaQ-N could well reproduce the observed discharge and stream Nitrate-N patterns and provided reliable spatio-temporal estimates of catchment N balance and networked in-stream N retention; (2) the two couplings were generally consistent with each other, while they showed subtle, but insightful differences in N transport (e.g., responses to small summer rainfall events) and transformations (e.g., the soil denitrification process), which could be attributed largely to different hydrological structures. Despite promising potential for further exploiting the coupled catchment modelling (e.g., combining in-depth uncertainty analysis), HiWaQ has the unique value of making better use of advanced hydrological modelling that embeds thoughtful modelling workflows and localised perceptual knowledge, thus better leveraging these advancements in the integrated catchment water quantity-quality assessments. We encourage interested researchers and modellers to contribute to further open-source development, which is oriented to be scientifically insightful and practically useful for catchment management.